The present invention relates to methods and apparatus for mapping color or gray scale values onto a monitor.
A typical display system consists of a computer, a frame buffer and a CRT display. The quality of the image produced in such a system is related to the number of pixels generated, the number of colors that can be assigned to each pixel and the type of processing done by the computer. A very crude system might display only 256 by 256 pixels with only two levels (black or white) for each pixel. A higher end system might display 512 by 512 pixels with 16 million colors (256 levels for each of red, green and blue) for each pixel.
The latter system is capable of displaying very high quality natural images captured from a TV camera or similar source. However, when using simple computer algorithms for generating synthetic images (consisting of text, lines, polygons, etc.) on such a system, the results often look little better than on the crude 256 by 256 pixel system due to the jagged and stair-stepped edges on some objects. These problems are called aliasing artifacts. Antialiasing algorithms can be used which eliminate many aliasing artifacts to produce smooth looking results. However, such algorithms often take excessive time for display generation, complicate other aspects of system design and sometimes "soften" the resulting image in an undesirable way.
A straightforward way to minimize aliasing artifacts without using antialiasing algorithms is to increase the number of pixels displayed. A display system having something in the range of 2000 by 2000 pixels to 4000 by 4000 pixels would suffer minimal image degradation due to aliasing artifacts. Implementing such a display system using prior art would require a frame buffer with between 8 and 24 bits per pixel for a total number of bits ranging from 32 million to 100 million or more bits. The preferred embodiment of the present invention acts as a 4000 by 2000 frame buffer using only 8 million bits.
For a display system which does not require a large number of colors, a color map may be used to reduce the size of the frame buffer required. Instead of storing a color value for each display location, an address for a separate color map is stored, with the address location containing the actual color value. However, for natural image displays which require a large number of colors (in the thousands) a color map is not as useful because the number of bits required for the color map address is comparable to the number of bits required for the color value itself, and the color map requires a larger amount of additional memory.
Dithering is one way to reduce the number of bits when the number of colors desired is too large for color mapping. Dithering involves alternating colors between adjacent positions so that the pattern appears to be a composite color to the human eye.
Some systems store the video image in YUV format rather than RGB format. In this format, Y corresponds to luminance information (brightness), U corresponds to the blue color difference value and V corresponds to the red color difference value. Thus, luminance information is separated from the red, green and blue colors. A linear conversion will convert the YUV information to RGB information. Because the luminance portion is separated out, this coding has advantages in some applications. Such a system will convert the YUV information back into RGB information before supplying the data to the CRT.